A low-pass filter (LPF) is a EQ filter that let's all low frequencies pass through unaffected, but cuts high frequencies. For this reason, it is also sometimes referred to as a high cut filter.
Most software EQ's will allow you to choose what frequency you want the filter to start at. Some will also allow you to configure the amount the frequency drops off below that point.
These filters are useful for darkening tracks. By taking away the brightness in tracks, tracks can be pushed back in the mix and not sound so up front. This can be useful for layering sounds and giving them depth.
A low-pass filter attenuating frequencies above 6 kHz